In the technical field dealing with high-frequency signals such as microwaves and millimeter waves, a directional coupler which divides such a high-frequency signal or combines such high-frequency signals is widely used. As an example of such a directional coupler, FIG. 1 of Non-patent Literature 1 illustrates a directional coupler including two post-wall waveguides sharing a waveguide narrow wall having an opening. FIG. 24 is a perspective view schematically illustrating a configuration of a directional coupler 7 disclosed in Non-patent Literature 1. To be precise, a configuration in which narrow walls of the rectangular waveguide illustrated in FIG. 24 are realized as post walls corresponds to the post-wall waveguide disclosed in Non-patent Literature 1.
As illustrated in FIG. 24, the directional coupler 7 includes a first rectangular waveguide 71 and a second rectangular waveguide 72. The first rectangular waveguide 71 and the second rectangular waveguide 72 share a narrow wall 73. The narrow wall 73 has an opening 731, and an inside of the first rectangular waveguide 71 and an inside of the second rectangular waveguide 72 are communicated with each other via the opening 731.
Provision of the opening 731 in the narrow wall 73 enables the first rectangular waveguide 71 and the second rectangular waveguide 72 to be electromagnetically coupled with each other. Accordingly, for example, in a case where a high-frequency signal is caused to enter a first port P1, the high-frequency signal is caused to exit not only from a second port P2 but also from a third port P3 and a fourth port P4. In this case, a ratio of a power of the high-frequency signal caused to exit from the third port P3 to a power of the high-frequency signal caused to enter the first port P1 depends on a strength of coupling between the first rectangular waveguide 71 and the second rectangular waveguide 72. The strength of coupling is referred to as a coupling degree. The coupling degree can be changed by changing a width W of the opening. In a case of a directional coupler having a coupling degree of 3 dB, a ratio of a power of the high-frequency signal caused to exit from the third port P3 to a power of the high-frequency signal caused to exit from the second port P2 is 1:1.